In what could be a major breakthrough in the quest for an effective AIDS vaccine, researchers at a U.S. government laboratory reported Thursday the discovery of two powerful antibodies that can prevent most strains of HIV, the virus that causes AIDS, from infecting human cells.

Scientists say the two antibodies, discovered in the blood of an HIV-infected individual, were able to neutralize more than 90 percent of known HIV strains in laboratory tests with human cells. Antibodies are the chemical proteins that the body makes to rally an immune system response following exposure to a virus or bacterium.

The problem with fighting off HIV infection is that millions of variants of the AIDS virus now exist around the world because HIV constantly changes its surface appearance to avoid detection by the immune system.

According to researchers at the Vaccine Research Center - part of the federally-funded National Institutes of Health, the two antibodies, called VRC01 and VRC02, interfere with HIV's ability to latch on to and cripple human T cells, the immune system's primary weapon against invaders.

Scientists discovered the antibodies in the laboratory using a novel technique that enabled them to identify areas on HIV's molecular surface that remain nearly constant across all viral strains.

One such area, called the CD4 binding site, is located on the surface spikes used by HIV to attach to human T cells and destroy them.

Gary Nabel, director of the Vaccine Research Center and the leader of the research team, says the VRC01 and VRC02 antibodies block HIV infection by attaching to the CD4 binding site.

"So, when the antibodies are directed against the virus, it reacts with that region, it inactivates the virus and the virus never has a chance to enter the cells that it would otherwise infect," he said.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which oversees the Vaccine Research Center, says the strategy used by Nabel and his colleagues reflects the intensified research efforts to block the AIDS virus at the earliest possible stage of infection.

"We need to focus on the vulnerabilities of the virus; how it makes that first infection and how it amplifies that infection," Fauci said. "Because if it infects one cell, and that cell dies, but doesn't infect another cell, the infection is over. It's only when the acceleration of one cell gets infected and then infects one, and sometimes two or three or four cells, once that happens, the horse is out of the barn and then you are really in a lot of trouble," he added.

Fauci says he's more optimistic than ever that scientists will be able to develop a vaccine against the AIDS virus. Encouraged by their discovery of the HIV-neutralizing antibodies, Vaccine Research Center director Gary Nabel and his colleagues share Fauci's optimism. Nabel says they have already begun to design a candidate vaccine to train the human immune system to make these potent proteins and for the first time, build a natural, protective wall against HIV infection.

"One of the reasons to be more optimistic now than we have been in the past is that we know it is possible to block infection with antibodies such as the ones that we have learned about in these studies. And if we can block HIV infection, then we really do have a chance of making a vaccine that could prevent disease," Nabel said.

Nabel estimates it will probably be three to five years before a vaccine is ready for human trials, which could take another five years. But he says there's always a chance that the drug works extremely well and trials are cut short to make the vaccine available sooner.

Two articles describing the discovery of HIV-neutralizing antibodies are published this week in the journal Science.